The Impact of Lead-Free Assembly on Base Materials

In addition, the advent of the European Union s Restriction of Hazardous Substances (RoHS) directive and the lead-free assembly processes that result are redefining the requirements for base materials. RoHS has a severe impact on all aspects of base materials technology. The impact of lead-free assembly on base materials and a method of selecting materials for lead-free assembly are discussed in Chaps. 10 and 11. Requirements to support circuit densification, reliability, and electrical performance are also critical and will be discussed in Chap. 9. This chapter discusses grades and specifications of base materials, as well as the manufacturing processes used to make them. 

Historically, the properties that received the greatest amount of attention were the glass transition temperature, Tg, and the coefficients of thermal expansion, or CTEs, particularly in the z-axis. With the advent of lead-free assembly processes, other properties have increased in importance as well.The most notable is the decomposition temperature,Ta.These properties were described in more detail in Chap. 6, and will be discussed again in Chap. 10, which focuses on the impact of lead-free assembly on base materials. However, some additional information as well as examples of the test data are included here, as well as comparisons of some common material types. 

THE IMPACT OF LEAD-FREE ASSEMBLY ON BASE MATERIALS... 

In addition to the works already cited, there have been excellent studies on the impact of lead-free assembly, specifically on PWB reliability. These works present statistical analyses showing the impact of lead-free assembly on PWB reliability and reach important conclusions regarding the base materials. Althongh there is not perfect agreement among all published works, the differences typically are the resnlt of a different focus— for example, whether the focus is on complex, thick PWBs with stringent reliability requirements versus relatively less complex PWBs with shorter intended field lifetimes or less stringent reliability requirements. Conclusions inclnde ... 

Chapter 10 discussed the impact of lead-free assembly on printed circuit boards (PCBs) and base materials, and also discussed many of the critical base material properties relevant to... 

Substantial work describing the impact of lead-free assembly temperatures on base materials and finished PWBs has been gathered. - " - These works have identified the critical base material properties that must be considered when selecting a material for lead-free assembly applications, though work on this subject is continuing. Table 10.2 provides a summary of these properties. 

Kelley, Edward, An Assessment of the Impact of Lead-Free Assembly Processes on Base Material and PCB Reliability, IPC/Soldertec Conference, Amsterdam, June 2004. 

Environmental legislation such as the European Union s Restriction of Hazardous Substances (RoHS) directive has a profound impact on all levels of the electronics supply chain, including these components. RoHS restricts the use of lead, which is an element in the solder used for component assembly onto printed circuits. The impact on the base materials and components is primarily the result of higher assembly temperatures that are associated with lead-free assembly. Table 7.1 summarizes the key issues for base material components. RoHS issues will be discussed further in Chap. 10. 

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